battery voltage with engine running 12.13V

[aki334]

Hi all, I am trying to diagnose low rpm and shaking at idle with ac on and when engine cooling fans turn on. This is only problem at stop lights.

 

My Legacy is 2017 so battery is about 4 years old and still original 356 cca.

 

My first question for you guys is:

 

Was your original battery group 25 with 356 cca? Every time I search group 25 it is at last 550cca

 

2. With engine idling in park can you guys measure voltage of your battery and report what you measure?

When I disconnected battery sensor voltage raised to 14.3 V which I think should be.

 

So I think I could have weak battery or bad battery sensor.

[ammcinnis]

Was your original battery group 25 with 356 cca? Every time I search group 25 it is at last 550cca

Legacys with 2.5 engines were originally delivered with the 356 CCA battery, while 3.6 engines got the 550 CCA. I believe that later Gen 6 Legacys were all fitted with the 550 CCA version, which is the only battery Subaru dealers stock for replacements.

 

When I disconnected battery sensor voltage raised to 14.3 V which I think should be.

What "battery sensor" are you talking about? Do you mean the current (and temperature) sensor attached to the negative battery post? It doesn't measure voltage.

 

I hope you don't mean that you disconnected the battery. Never disconnect the battery while the engine is running! Doing so can damage the car's electrical system; the battery serves an essential purpose of leveling voltage spikes in the system.

 

So I think I could have weak battery or bad battery sensor.

The resting voltage of a fully charged battery should be about 12.6 volts (~2.1 volts per cell x 6 cells). A resting voltage of ~10.5 volts could indicate a shorted cell, which a load test of the battery should also reveal.

 

My Legacy is 2017 so battery is about 4 years old and still original 356 cca.

Based on age alone, it's probably time to spring for a new battery. Any BCI Group 25, 34 (not 34R!), or 24 (not 24F!) battery will be a drop-in replacement. Search this forum for a thread on battery replacement choices: https://legacygt.com/forums/showthread.php/gen-6-battery-replacement-info-278179.html?t=278179 ... but then you knew about that already. Edited May 4, 2021 by ammcinnis

[macone]

At least the 2005 Legacy has a feature to disconnect the alternator clutch when there is a "low-load situation" detected. Does the battery get charged at all or is it always empty?

[aki334]

The car starts fine, and I tested battery voltage while the engine was idling in park.

It was giving me 12.13V then I just disconnected the "small two pin sensor on the negative cable" (temperature/current sensor) and voltage increased to 14 plus Volts.

 

If somebody has DVOM/multimeter to just confirm this form me please, I wonder what will be your battery voltage with engine idling in park.

[bsheaffer]

The alternator doesn’t turn on unless it needs to, it does not run all the time for these. Also your battery is pretty old… that would be probably the first place to look….

[waldguy]

Run a OBDII bluetooth scan tool while operating and read the volts. The alternator kicks in when the car is decelerating.

 

Historically, in some driving situations (eg. lots of highway), it wasn't enough to bring the car's battery to full charge.

 

In my situation, the battery was always around 12.15 volts or so.

 

TSB Number:11-174-17R addresses the issue (Aug 8, 2017) to ramp up the charging in normal driving conditions.

 

My original battery lasted 5 years, then I replaced it with an AGM. Never had a problem with the original but it definitely seemed sluggish starting in the cold.

[aki334]

Run a OBDII bluetooth scan tool while operating and read the volts. The alternator kicks in when the car is decelerating.

 

Historically, in some driving situations (eg. lots of highway), it wasn't enough to bring the car's battery to full charge.

 

In my situation, the battery was always around 12.15 volts or so.

 

TSB Number:11-174-17R addresses the issue (Aug 8, 2017) to ramp up the charging in normal driving conditions.

 

My original battery lasted 5 years, then I replaced it with an AGM. Never had a problem with the original but it definitely seemed sluggish starting in the cold.

 

 

Thank you for confirming this for me.

I have just read somewhere that there is a lawsuit against Subaru for bad battery/charging system problems for models 2018 and up.

Your answer confirms same voltage as I measured which tells me Subaru did not do this right if you are constantly doing short trips. I will keep my battery sensor disconnected from now on since engine idle improves and battery voltage reads 14.3V instead of 12.1V. I will also get a new battery as others have suggested and will still keep the sensor disconnected because of the short trips.

[ammcinnis]

I will ... still keep the sensor disconnected because of the short trips.

Just be aware that doing so runs the risk of overcharging the battery. It's generally not a good idea to disconnect something unless you have a thorough understanding of why it's there and how it's supposed to work, and from your posts here I don't think you yet understand how "the sensor" is used by the battery management system. Your car, your choice, though.

[aki334]

Just be aware that doing so runs the risk of overcharging the battery. It's generally not a good idea to disconnect something unless you have a thorough understanding of why it's there and how it's supposed to work, and from your posts here I don't think you yet understand how "the sensor" is used by the battery management system. Your car, your choice, though.

 

Hi Sir,

 

I do not know much except that I worked on the cars almost all my life and about 26 years professionally. Yes I know very well what it does. Measures current, voltage and temperature to tell alternator when to turn on and when to basically free spin. On some more expensive cars it does more than that. Anyway, when alternator kicks on then we have Lenz's Law theory going on and if you ever did that experiment you will know what I am talking about. But in short when alternator kicks on the pulley of the alternator will be much more difficult to spin which causes more power loss for the engine and that is when battery gets charged. Again here we have the theory called "you can not get something for nothing" I am sure there is voltage regulator on the alternator that will prevent overcharging. If this was AGM battery then I would not worry as much since it can take discharging and recharging better than the standard battery.

 

 

 

Just because I asked the question certain way you can not judge my knowledge.

 

So one question to you Sir, if Subaru knew what they were doing why is there TSB for the charging system and why is there now law suit for premature battery failures? I am sure those people did not disconnect battery sensor like I plan on doing.

 

 

Please read this:

https://www.torquenews.com/1084/update-subaru-outback-ascent-battery-drain-lawsuit-what-you-should-know

Edited May 9, 2021 by aki334